The invention relates broadly to a tool for controlling flow of fluid from a tubing string into a low pressure earth formation. More specifically, the invention covers a tool capable of holding the fluid in a tubing string, in a formation having a bottom hole pressure less than the hydrostatic pressure of the fluid in the string, until a predetermined pressure is applied to the tool to release the fluid.
When a wellbore is drilled into earth formations which have low fluid levels, the bottom hole pressure in the wellbore will be very low. This low pressure condition in the well casing can create a problem with regard to treating the formation with acids or other treating fluids. For example, if the pressure at the face of the formation to be treated is less than the hydrostatic pressure of the fluid in the tubing string at that depth, a means must be provided to hold the treating fluid in the tubing string until it reaches the zone to be treated. Otherwise, the low pressure condition in the wellbore will allow the treating fluid to "gravity flow" out of the tubing string.
Various attempts have been made to solve the problem described above. The closest prior art we are aware of is described in U.S. Pat. No. 4,050,516, granted to Dresser Industries Inc. The invention described in this patent is a spring-loaded valve, which is fastened to the end of the tubing string. In the valve device is a flow chamber which communicates with the well casing, to allow fluid to flow into the formation.
During run-in of the tubing string, the force of the spring keeps the valve head seated in an access opening at the bottom of the flow chamber. Part of the fluid in the tubing string bypasses the flow chamber and enters a pressure balancing chamber below the valve head. The fluid in the pressure balancing chamber thus exerts an upward pressure against the opposite end of the valve head. The force of the spring and the pressure exerted by the bypassing fluid are able to overcome the hydrostatic pressure of the fluid against the valve head and thus prevent the fluid from entering the flow chamber. To release fluid through the flow chamber and into the formation, a sufficient amount of pressure is applied against the fluid in the tubing string to overcome the upward force against the valve head.